The present invention relates to a method of improving the performance of solar cells. In particular, the invention relates to a method of removing electrical shorts and shunts created during the process of fabricating such solar cells.
The voltage and current output of thin film solar cells, such as amorphous silicon solar cells of the type described in U.S. Pat. No. 4,064,521 entitled SEMICONDUCTOR DEVICE HAVING A BODY OF AMORPHOUS SILICON which issued on Dec. 20, 1977 to D. E. Carlson, may be greatly reduced or completely eliminated by the presence of electrical shorts or shunts formed during the process of fabricating such solar cells. Electrical shorts occur either when there is a hole, such as a pinhole, in the semiconductor body which causes the front and back electrodes to touch or if there is a conductive metal which extends through the semiconductor body.
A shunt is the loss of charge in the semiconductor body due either to an imperfect barrier formation or to the formation of an ohmic contact by a high work function metal rather than a Schottky barrier. The problems created by solar cell defects which cause shorts or shunts are greatly increased with increased solar cell size.
In order to economically fabricate large area solar cells, either methods to eliminate such shorts and shunts during fabrication or methods of removing such defects after fabrication must be developed. In U.S. Pat. No. 4,166,918 entitled METHOD OF REMOVING THE EFFECTS OF ELECTRICAL SHORTS AND SHUNTS CREATED DURING THE FABRICATION PROCESS OF A SOLAR CELL which issued on Sept. 4, 1979 to G. E. Nostrand et al., a method for removing the effects of electrical shorts and shunts in large area solar cells is described. In accordance with that method, there is used a reverse bias voltage of sufficient magnitude to burn out the defects in the solar cell, but of a magnitude less than the breakdown voltage of the cell.
It has now been found that the method described in the U.S. patent issued to Nostrand et al., while successful in many cases, can be improved upon.